1. Field of the Invention
This invention is related to methods for determining the rate of penetration of a drillbit and using the determined rate of penetration for controlling the operation of downhole logging tools. The method of the invention is applicable for use with both measurement-while-drilling (MWD) tools and wireline tools.
2. Description of the Related Art
In the rotary drilling of wells such as hydrocarbon wells, a drill bit located at the end of a drill string is rotated so as to cause the bit to drill into the formation. The rate of penetration (ROP) depends upon the weight on bit (WOB), the rotary speed of the drill and the formation and also the condition of the drill bit. The earliest prior art methods for measuring ROP were based on monitoring the rate at which the drill string is lowered into the well at the surface. However because the drill string, which is formed of steel pipes, is relatively long, the elasticity or compliance of the string can result in the actual ROP being different from the rate at which the string is lowered into the hole.
U.S. Pat. No. 2,688,871 to Lubinski and U.S. Pat. No. 3,777,560 to Guignard teach methods to correct for this difference by modeling the drill string is as an elastic spring with the elasticity of the string being calculated theoretically from the length of the drill string and the Young""s modulus of the pipe used to form the string. This information is then used to calculate ROP from the load applied at the hook suspending the drill string and the rate at which the string is lowered into the well. These methods do not account for the friction encountered by the drill string as a result of contact with the wall of the well. Patent FR 2 038 700 to Gosselin teaches a method of correcting for this effect by making an in situ measurement of the modulus of elasticity. This is achieved by determining the variations in tension to which the drill string is subjected as the bit goes down the well until it touches the bottom. Since it is difficult to determine exactly when the bit touches the bottom from surface measurements, strain gauges are provided near the bit and a telemetry system is required to relay the information to the surface. In MWD applications, the data rate of the telemetry system is necessarily limited. Additionally, this method still does not provide measurements when drilling is taking place.
There have been a number of teachings of the use of Kalman filtering for determining the rate of penetration of a drillbit. For example, Sengbush (FR 2 165 851 and AU 44,424/72), uses a mathematical model applicable for roller cone bits for describing the drill bit cutting rate. The model requires a knowledge of the drill depth, the drill rotational speed, and the weight on bit. Chan in U.S. Pat. No. 5,551,286 discusses a related problem of a wireline logging tool on an elastic cable.
In U.S. Pat. No. 4,843,875 to Kerbart, during an initial period, the well is drilled keeping, on average, the value of weight F of the drill string measured at the surface relatively constant, and the instantaneous values of the drill string rate of penetration VS and the weight F are measured at the surface at different successive moments. The value of the drill string average rate of penetration VSM at the surface is determined from the values of VS measured and the successive values of dF/dt of the first derivative with respect to time. The coefficient of apparent rigidity of the drill string during the initial period is then determined from the values of VSM, VS and dF/dt. Finally, the rate VF is calculated. In U.S. Pat. No. 5,551,286 to Booer, a state space formulation of the model in the Kerbart patent is used with a Kalman filter to determine the downhole ROP. The quantity observed in Booer is the surface displacement. Those versed in the art would recognize that a fundamental problem in Kalman filtering is the identification of the state transition matrix that governs the evolution of the state space model. Kalman filtering is also computationally intensive.
U.S. Pat. No. 5,585,726 to Chau teaches the use of a three-component accelerometer near a drillbit used for boring a near horizontal borehole. Integration of the accelerometer outputs is performed to determine the position of the drillbit. This integration is susceptible to integration errors. In Chau, at specified times, a dipole antenna is used in conjunction with a surface EM transmitter to get an absolute position of the drillbit and to correct for the integration errors. This is possible in near horizontal borcholes but is impractical for deep wells drilled in hydrocarbon exploration.
Determination of the ROP is of particular importance in measurement of compressional and shear velocities of formations in measurement-while-drilling (MWD) tools. In wireline logging, a plurality of acoustic transmitters is used in conjunction with arrays of acoustic receivers for determining these velocities, the transmitters being excited at regular intervals related to the logging speed to give redundant measurements of these velocities. In MWD applications using devices such as that described in U.S. Pat. No. 6,088,294 to Leggett et al, the contents of which are incorporated herein by reference, excitation at regular time intervals is not necessarily desirable if the ROP is time varying. The method of the present invention makes it possible to determine the ROP with relatively simple computations and thus control the operation of the acoustic logging tool.
Generally, depth determination is less a problem in wireline tools. One of the earliest teachings is that of Bowers et al (U.S. Pat. No. 3,365,447) In Bowers, the tension between the tool and its supporting cable is measured, as is the movement of the cable at the surface of the earth. The tension and cable movement are then combined in a computer along with a plurality of constants representative of various characteristics of the cable and its surround medium to produce an output signal representative of the movement of the tool and relating to the changes in tension Examples of the use of accelerometers for wireline use are given in Chan (U.S. Pat. No. 4,545,242) teaches a high resolution method and apparatus for measuring the depth of a tool suspended from a cable. The tool includes accelerometers for measuring its acceleration and this measurement is combined with a cable depth measurement with which the amount of cable in the borehole is determined. A Kalman filter is employed to continually provide estimates of the velocity and depth of the tool from the accelerometer and cable depth measurements. A filter modifier alters operation of the filter during discontinuous motions of the tool such as when it is stuck and slips. A tool sticking detector senses when the tool is stuck and for how long to correspondingly modify the filter by forcing it to more strongly rely upon accelerometer measurements when the tool is stuck and gradually return to normal filter operation when the tool resumes movement after having been stuck. However, as noted above, it is particularly when a tool is stuck that integration of accelerometer measurements tend to become unreliable.
There is a need for a method of determination of depth of a tool in a borehole that is not susceptible to the errors discussed above. The present invention satisfies this need.
The present invention is a method of determining the rate of penetration of a downhole drilling assembly conveyed in a borehole during drilling of the borehole. An accelerometer on the downhole assembly is used to make measurements indicative of axial motion of the drilling assembly. In one embodiment of the invention, these measurements are used to determine the axial velocity of motion. Maxima or minima of the velocity are identified and from these, the rate of penetration is determined assuming that the penetration occurs in discrete steps. Alternatively, maxima or minima of the axial displacement are determined and these are used to obtain a depth curve as a function of time. In an alternate embodiment of the invention, the rate of penetration is determined from the average acceleration of the downhole assembly and its instantaneous frequency. The determined rate of penetration may then be used to control the operation of a logging while drilling tool. Specifically, the activation of a transmitter of the logging tool is controlled to give measurements at desired depths. This is particularly desirable in array logging tools such as are used in borehole-compensated acoustic logging. Operation of other downhole tools may also be controlled based on depth determination.
In an alternate embodiment of the invention, measurements made using accelerometers are also used to get an estimate of the depth of a downhole tool conveyed on a wireline.